1. Technical Field of the Invention
This invention is related to air guns in general and in particular, to the art of measuring the instant that the air gun creates an acoustic wave for use in seismic exploration.
2. Technical Description of the Prior Art
Air guns are widely used as sound sources in marine seismic exploration. A typical air gun is fired electrically by actuating a solenoid valve which, in turn opens a main valve. When the main valve opens, jets of compressed air are emitted through one or more exhaust ports to generate the desired acoustic pulse. Due to mechanical irregularities in gun construction, there is usually an irregular time delay between the time the solenoid is actuated and the time that the gun actually fires. Furthermore, no two air guns will necessarily exhibit the same free-time delay. Accordingly, various stratagems have been devised for determining the actual air-gun fire-time.
Many operators secure a hydrophone externally to the gun, some distance away from the gun exhaust ports. The first impulse detected by the hydrophone is considered the firing time. Other workers in the art measure the pressure-drop in the compressed air-line to the gun that occurs when the gun is fired. An example is disclosed in U.S. Pat. No. 3,985,199. In U.S. Pat. Nos. 4,034,827 and 4,047,591, a magnetic sensor is used to detect the instant that the main valve moves to open the exhaust ports. In co-pending U.S. Patent Application Ser. No. 749,547, assigned to the assignee of this invention, a pressure sensor is mounted inside the firing chamber of an air gun to detect a pressure change when the gun fires.
All of the above-described devices and many others of similar ilk are not completely satisfactory for reasons well known to geophysicists. Accordingly, we disclose a simle, sturdy, inexpensive device to determine the firing time of an air gun.